There are a number of techniques which are known for pasteurizing and processing liquid egg. The more popular approaches involve the use of conventional plate heat exchangers, steam infusion systems, or combinations of both. However, these techniques have limitations because of the physical nature of liquid egg. Plate heat exchangers are very popular in that they are very efficient and relatively simple to use. However, plate heat exchangers are limited in the amount of heat that they can convey to liquid eggs without causing irreparable harm such as undue coagulation, scorching, caking on the plate, and the like. Furthermore, the highest effective temperature range over which heating can be accomplished by plate heat exchangers is limited to between about 150.degree. F. to about 155.degree. F., particularly for long term, continuous processing techniques. As a matter of practical reality, plate heat exchangers can only be used to heat liquid whole egg to pasteurization temperatures of between about 140.degree. F. and about 155.degree. F. for an extended period of time.
Steam infusion systems can be used to obtain much higher temperatures such as, for example, 165.degree. F. However, it is necessary to separate the condensed steam and residual water from the liquid egg during processing. The equipment used for steam pasteurizing liquid eggs is also quite expensive and elaborate.
It is of course possible to pasteurize at lower temperatures without the attendant shortcomings of high temperature methodologies. However, in so doing, sacrifices are made in terms of the degree of microbial kill and extent of the storage stability of the resulting liquid egg product
Electroheating has been used successfully for the heat treatment, and principally the cooking, of food stuffs. A particularly important technique for electroheating is described by Mr. David Reznik, (a co-inventor hereof) in U.S. Pat. No. 4,739,140, the entirety of which is hereby incorporated by reference. Mr. Reznik found that when an AC electric current at a frequency exceeding mains frequency is passed through food, heating of the food can be accomplished without substantial electrolysis or reaction between the food product and the electrodes.
Mr. Reznik's '140 patent does not describe, however, pasteurizing egg. Furthermore, this important work does not discuss any of the unique qualities or unique problems associated with the continuous pasteurization of liquid egg. Liquid egg is unique and when considering its preparation, processing and/or storage a number of often competing criteria must be taken into account, including, without limitation: storage stability, content of potentially pathogenic bacteria, foaming ability, emulsion properties, viscosity, detrimental coagulation, and pourability. Largely because of this complexity and because of practical commercial considerations, the application of Mr. Reznik's original discovery to continuous liquid egg pasteurization was not without difficulty. Originally, such problems as caking on one or the other electrode (particularly at low frequency), detrimental coagulation, odor, and arcing, were observed. The reasons for these problems are not yet completely understood. However, the interplay of the demands of regulatory oversight, commercial desirability, the qualities and properties of liquid egg, and the physical constraints of the original electroheating apparatus design are most likely responsible.
U.S. Pat. No. 4,695,472, (the "'472 patent") discusses methods and apparatus for extending the shelf life of fluid food products including eggs. The methods and apparatus described involve the repeated application of high voltage, high current density, discrete electric pulses to fluid food products. Field strengths used are, at a minimum, 5,000 volts/cm, voltages as high as, for example, 37,128 volts are disclosed. Current densities of at least about 12 amps/cm.sup.2 are also disclosed as are pulse frequencies of between 0.1 and 100 Hz. Preferably, the treatment methods according to the '472 patent involves the application of at least 2 and more preferably at least about 5 high energy pulses to the material being treated. The principal embodiments disclosed in the '472 patent involve the use of direct current. The techniques described in the '472 patent have several disadvantages. Most notably, electrolysis is caused with resultant electrode loss, contamination, and, at least in the case of eggs, detrimental coagulation. To mitigate this problem, the '472 patent requires the use of membranes interposed between the electrodes and the food product being treated. The '472 patent also fails to appreciate that highly functional egg having outstanding storage stability can be obtained without the need for the complicated pulsed electrical treatment disclosed therein.
Most importantly, the '472 reference does not appreciate the difficulties attendant the continuous pasteurization of liquid egg. Specifically, nothing in the '472 patent discusses the problems attendant ensuring the absence of detrimental coagulation. Most notably, even though the '472 patent does describe arcing as a potential problem, the only effect attributed to such arcing is damage to the electrodes. Furthermore, all of the tests conducted in accordance with the '472 patent of liquid egg involve the use of static, bench testing devices at peak voltages of 34,000 volts or higher and currents ranging from between 7,200 and 14,200 amps. No continuous egg processing was undertaken. As will be described further herein, when similar static testing was undertaken by the present inventors using the technology described herein, totally coagulated egg resulted.
To date, the '472 patent has, apparently, not been the subject of commercial use nor are the present inventor(s) aware of any egg product being produced in accordance with its teaching. Thus, whether or not a sufficiently functional egg product can be produced with such methodologies and apparatus is unknown.